1. Field of the Invention
The present invention relates to an air quantity control device of an internal combustion engine and in particular to an air quantity control device of an internal combustion engine that detects an abnormality in a throttle actuator or the like.
2. Description of Related Art
Conventionally, there has been proposed a technology comparing intake air quantity, which is calculated based on a throttle opening, and intake air quantity, which is measured with an airflow meter or the like, thereby detecting an abnormality in the airflow meter or a throttle actuator constituted by a throttle valve, a throttle position sensor and the like (for example, refer to Patent document 1: JP-A-2006-257939). The technology described in Patent document 1 estimates the intake air quantity at least from atmospheric pressure, engine rotation speed and the throttle opening and determines that the airflow meter is abnormal when a deviation between the estimated air quantity and the actual air quantity measured with the airflow meter is equal to or larger than an abnormality determination value and an air-fuel ratio is outside a predetermined range. It is thought that not only the abnormality in the airflow meter but also the abnormality in the throttle actuator can be detected with the technology of Patent document 1.
In recent years, against a backdrop of the concern about the depletion of the petroleum resources, the necessity for the alleviation of the global warming and the like, the alcohol fuel has been drawing more and more attentions as a substitute for the fossil fuels such as the gasoline. The alcohol fuel is used singularly or is mixed with the other fuel such as the gasoline and used as the fuel for the internal combustion engine. Against such the backdrop, various kinds of methods for performing the combustion control of the internal combustion engine using the fuel including the alcohol have been proposed (for example, refer to Patent document 2: JP-A-H1-113528).
The fuel property differs between the gasoline and the alcohol. For example, the octane number of the alcohol is higher than that of the gasoline. The ignition timing can be set on a more advanced side as the octane number of the fuel increases. Therefore, in the case where a mixture gas containing intake air quantity defined at a predetermined throttle opening is combusted at a target air-fuel ratio, it is thought that the output torque of the internal combustion engine generated by the combustion is larger in the case of the alcohol than in the case of the gasoline because of the difference in the ignition timing. Therefore, in the case where the abnormality in the throttle actuator or the like is detected based on the result of the comparison between the intake air quantity estimated from the throttle opening and the like and the intake air quantity measured with the airflow meter and the like, the output torque at the time when the abnormality is determined to be present differs in accordance with the fuel property. As a result, in the case of using the fuel containing the alcohol, there is a possibility that the occurrence of the abnormality is not detected although there is actually an occurrence state of the excess torque or there is a possibility that the occurrence of the abnormality is detected although there is actually no occurrence state of the excess torque.